What is Sovereign Car Plate?

Can anybody please explain what a sovereign car or sovereign car plate is?

This should explain it far better than I and explains the meaning of " sovereign plates and cars " - www.wired.com/autopia/2009/06/dmv-is-diy/ (http://www.wired.com/autopia/2009/06/dmv-is-diy/)

Thanks a lot, I guess it clarifies the issue somewhat. I would presume there should be some legitimate sovereign plates too.